bpo-39761: Fix dtrace build with empty $DFLAGS (GH-18766)

This fixes a regression introduced in bpo-38960.

When DFLAGS was empty, "$DFLAGS" results in an empty argument ("").
Without the quotes, an empty variable will be ignored by the shell.
diff --git a/configure b/configure
index d1ec709..893d0e1 100755
--- a/configure
+++ b/configure
@@ -11487,7 +11487,7 @@
 else
               ac_cv_dtrace_link=no
             echo 'BEGIN{}' > conftest.d
-            "$DTRACE" "$DFLAGS" -G -s conftest.d -o conftest.o > /dev/null 2>&1 && \
+            "$DTRACE" $DFLAGS -G -s conftest.d -o conftest.o > /dev/null 2>&1 && \
                 ac_cv_dtrace_link=yes
 
 fi